The invention relates to improvements in mechanisms and methods for calcining cement including a precalciner, a calcining kiln, cooler and a burner and gas flow control means. More particularly, the invention relates to an improved cement calcining construction including a suspension type heat exchanger with a calciner, a rotary tubular kiln and a cooler connected thereto with a conduit for the discharge of material from the kiln to the cooler and a tertiary air conduit for supplying air to the precalciner.
For convenience of the description of the method and mechanism, the terms "primary air", "secondary air" and "tertiary air" will be used herein, and these terms will be defined inasmuch as they are not universally employed in the same manner in literature referring to cement calcining structures. What is meant by "primary air" is the air which is supplied to a burner which is usually the principal burner of the rotary tubular kiln for processing the air fuel mixture, and the air is usually supplied as atomizer air to the burner.
What is meant by "secondary air" is the combustion air delivered to the rotary tubular kiln which is required by the burner flame cone for complete combustion.
What is meant by "tertiary air" is that combustion air which is required in the precalcining operation for the predominantly flameless precalcination deacidification process of the ground rock which is being processed into cement.
The primary air is prepared and delivered by a compressor or blower having an atmospheric air intake, or the primary air may be obtained from a preheating aggregate. The primary air is compressed and supplied to the burner of the rotary tubular kiln as processing air. The secondary air is usually introduced from coolers into the rotary tubular kiln which utilizes the secondary air for the firing or calcinination.
The tertiary air is supplied by a tertiary air conduit which bridges the rotary tubular kiln and is connected to receive air from the cooler and is supplied to the precalcining zone or to the precalciner where it is mixed with fuel and is delivered in suspension with the ground rock to be deacidified and heated to approximately 700.degree. through 800.degree. C., and supplies the thermal energy necessary for the endothermic deacidification process.
A device for calcining cement of the type above referred to is shown in the German OS No. 2,360,580. Therein, the tertiary air conduit is referred to as "secondary air conduit", shown at 82 in accord with the above definition. It is stated on page 5, paragraph 2 of the German OS that the air velocity in the secondary air conduit 82 is higher than the gas velocity in the rotary tubular kiln for economy reasons. In this known device, a dust collector is shown at 81 and is built into the secondary air conduit 82, and for this reason, this structure exhibits a considerably higher resistance than the flow system of the secondary air including the flame gases in the rotary tubular kiln. Consequently, the kiln and gas flow system is equipped with a constricted opening shown at 83 in order to achieve a matching of the resistances in the two flow systems. However, a blower which produces the matching of the resistance can be provided in the secondary air conduit 82 instead of the constricted opening 83.
The latter solution, however, as expressly stated, does not represent an expedient method due to a short useful life because air at high temperature and mixed with clinker dust is conveyed.
Another known device of the same species wherein a throttle device is also disposed in the flue gas channel between the intake of the rotary tubular kiln and the lowest cyclone of the suspension type heat exchanger or precalciner is disclosed in German OS No. 2,325,468. In this structure, it is a matter of utilizing a pivotal throttle valve.
In the known arrangements wherein the throttle location or a throttle member is used in the area of the kiln intake in order to balance the air flow, or respectively, the resistances of the flow systems of the rotary kiln and the tertiary air conduit introduce a series of disadvantages.
Because the temperature level is very high, particularly in a short rotary tubular kiln, for example between 1,000.degree. and 1,200.degree. C., the incorporation of movable throttle members is difficult and the useful life is unsatisfactory.
Further, vortex flow patterns occur at the throttle location producing injurious dust circulations and end in the result that the kiln output and heat consumption deteriorate noticeably.
Further, incrustations which may progress to such a degree that a shut-down of operations is forced, are formed under the influence of the disturbed flow given corresponding temperature levels and given materials which tend to soften.
It is an object of the present invention to achieve a flow equalization between secondary air and tertiary air without the kiln operation being disadvantageously influenced due to the formation of disturbed flow zones in the area of the kiln intake, and in particular while avoiding incorporation of throttle elements in this area. Further, an object is obtained of a sensitive, very precise control of the flow system.
The objects of the invention are achieved in that with a given device for burning cement of the type above referred to which comprises a suspension type heat exchanger with a precalciner, a rotary tubular kiln and a cooler connected thereto by means of a vertical conduit, as well as including tertiary conduit means bridging the rotary tubular kiln, a device for increasing the resistance of the gas flow is disposed in the conduit. This flow resistant device can either be provided as a matter of a fixed throttle element, such as a constriction of cross-section as well as an adjustable throttle member such as a variable flap or slide.
With the arrangement in accordance with the principles of the invention which attain resistance increase, such device is situated at a location with a significantly lower temperature so that the useful life is increased under conditions which are otherwise identical with devices heretofore available.
A further advantage is obtained in that turbulence generating installations are avoided in the area between the kiln intake and the suspension type heat exchanger whereby the inclination to form dust circulations and injurious deposits is avoided.
A further feature of the invention is that with the provision of a variable choke member, an extremely precise and uniform control of the amount of combustion air is achieved by means of balancing secondary air and tertiary air whereby temperature transgressions and irregularities in clinker burning are avoided.
A further feature of the invention is the attainment of a reduction of temperature achieved by means of creating better flow relationships in the intake chamber whereby undesired material solidifications and cakings are avoided.
In accordance with the invention, a vertical shaft or conduit between the rotary kiln and the cooler branches into two separate channels with one connecting the cooler to the rotary kiln, and the other connecting the cooler to the tertiary air conduit with a device for increasing the resistance of gas flow being disposed in the passage between the cooler and rotary kiln.
With the foregoing arrangement, the advantage is obtained in that the conduit which connects the cooler to the tubular kiln can be constructed of a relatively narrow cross-section so that the variable constriction, such as a throttle member which is disposed at this location can be positioned without coming into contact with the heated descending clinker and yet can achieve a significantly high flow resistance in order to exercize the intended throttle effect.
It is a feature of the invention that the throttle member be designed as an adjustable sliding plate which is operated with a motor drive. A further throttle member can also be disposed in the branch of the conduit leading between the cooler and the tertiary air conduit.
In order to reduce the amount of dust arising in the cooler before the entry of the air leaving the cooler and flowing into the tertiary air conduit, a channel is located which is designed as a dust separator and is equipped with a flow guide member inducing a turbulent flow around the mouth of the tertiary air conduit.
A further feature of the invention is that one wall of the channel connecting the tubular kiln and cooler is offset in the direction of the movement of the lower side of the tubular kiln, being offset relative to a vertical plane extending through the axis of rotation of the tubular kiln and extending through the constricted cross-sectional area which increases flow resistance from the cooler to the kiln. The sliding throttle plate is expediently located in this constricted cross-sectional area.
An advantage which derives from this throttle member and its location is that the conduit constriction is increased by the throttle member and the design of the walls is such that the descending heated clinker cannot contact or damage either the walls or the throttle members.
Other objects, advantages and features will become more apparent as will equivalent structures and methods which are intended to be covered herein, with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings, in which: